Can I change from stock 175/65R15 tires to 195/65R15?

I don't know about right size for Prius C tires, but used tires that don't match required size in the long run will costs huge in terms of bad gas mileage. SimpleTire.com sells lowest priced new tires online and a good pair of Low Rolling Resistance (LRR) tires with higher than normal pressure/PSI will improve your MPG so much that the money you save on gas will pay for most of the cost of them over their lifespan.

 

Check this out.
Tire Size Comparison

 

Stick to the stock size. Fit issues aside, according to the calculator above: the prospective tires have 4.2% larger Outside Diameter. That's the number that your "gearing", odometer and so on will be thrown off by.

For a little more fun: if you ever have a flat and need to put the temp spare on: possibly the car will throw a code due to the variation in OD?

 

Not enough space?

 

On my manual Hyundai Accent, I switched from 175/65/14 to 195/65/15 wheels/tires, that I took from our bigger Elantra. Best thing I ever did for my Accent. Dropped rpms by 250 on the highway, bumps are softened, cornering & handling are superior & my MPG..... went up. The homely Accent got its first compliments, too.
Also went to bigger wheels/ tires on the Elantra, with good results.....& more compliments.

 

Have mine upsized from stock 175/65/15 to 185/65/15 on the same rims, dropped 3 psi on all corners and it rides like a Camry. My regular tire man opposed to that idea and so went to the other one that was willing to do it. Been that way for over 18 months without any problems.

 

Lowering pressures and going with a wider-than-stock tire is going to impact mpg some? It'll also throw the odometer off some, making the mpg "look" worse.

 

There is no doubt that the stock size (175/65R15) is the best option. But if you are living in a country where speed breakers are not made as per rules then Prius C body will touch, often, with the higher speed breakers. Dia of 195/65/R15 is 1.02 inches bigger than the stock size and it will solve this problem to some extent.
On the other hand, there will be a bit higher fuel consumption and the actual speed will be a bit higher than the given on the speedometer. This is due to the wider tire size (0.79 inch bigger) and bigger tire diameter respectively.

 

That's correct, but radius increase is only half that amount, roughly 1/2"; that's the amount the car is raised.

 

175 to 195 is 20 millimeters wider....that is 3/4 of an inch wider...now the 65...65% of 175 is 113.75 millimeters and 65% of 195 is 126.75 millimeters so the tire is 13 millimeters taller also (which is half an inch taller)....so the difference between a 175/65 and a 195/65 is a tire 3/4 of an inch wider and half an inch taller (total of an inch in the wheel well-0.5 inch below the wheel and 0.5 inch above the wheel)...those numbers shouldnt cause trouble inside the wheel well, but the question is...will the 195 seat or stay seated on the wheel without breaking the bead...

 

You can also just apply the .65 factor to the difference of the two widths, gets the same conclusion:

195-165=20
20x.65=13